1. Field of the Invention
The present invention generally concerns fluoropolymeric compositions. In particular, the invention concerns an improved composition that is isolated from an aqueous blend of fluoroelastomer and microparticulate fluoroplastic materials. The improved composition is particularly useful as a manageable intermediate in the development of microfiber-reinforced fluoropolymeric components.
2. Description of the Prior Art
Polytetrafluoroethylene (PTFE) is in many respects an unusual polymer. It is exceptional in its chemical inertness as a result of the strength of its carbon-fluorine bonds and shielding of its carbon--carbon bonds by the bulky fluorine atom. PTFE is exceptionally useful for high temperature applications because it has a high melting point and remains chemically inert at high temperatures. In addition, PTFE's unusually low frictional coefficient, surface free energy, and dielectric constant all testify to its unusual morphological structure. While these extremely attractive properties cause PTFE to be useful in a broad array of end use applications, they also lead to an unusual set of problems in characterizing some properties of PTFE as well as to difficulties in processing compositions based on PTFE.
The inertness and insolubility of PTFE make it virtually impossible to characterize the molecular weight of a PTFE component by direct, conventional means such as osmometry. The prior art typically resorts to indirect means, such as the determination of specific gravity after recrystallization from a melt at a controlled rate of cooling, as an indicator of molecular weight. The higher the molecular weight of the PTFE, the longer its chain length and the more difficult it is to recrystallize to a highly ordered (crystalline) and, therefore, dense structure. Consequently, the specific gravity of PTFE at any given crystallinity level and specified thermal history is an indirect measure of molecular weight. Crystallinity may be independently assessed by X-ray crystallography or calorimetry, and the specific gravity obtained upon cooling (recrystallizing) a PTFE melt at a prescribed rate (referred to as the standard specific gravity (SSG)) is a commonly employed measure of molecular weight.
It is well established that certain physical behavior of PTFE is a strong finction of molecular weight and crystallinity (Blair, John A., Fluorocarbons, Polymers, "Encyclopedia of Industrial Chemical Analysis," vol. 13, pps. 73-93). For example, most commercial molding powders of PTFE have a very high molecular weight corresponding to an SSG of between about 2.16 and 2.25, depending on crystallinity. High molecular weight is needed to develop adequate tensile strength and the elongation required for typical end uses of an essentially waxy polymer.
At lower molecular weight, PTFE becomes very weak and brittle while retaining its low coefficient of friction. Low molecular weight PTFE is typically a friable powder, which can be very highly crystalline, and enjoys use as a dry lubricant.
An important distinction in behavior between low molecular weight and high molecular weight PTFE lies in the propensity of the high molecular weight PTFE to fibrillate when in its highly crystalline, as-polymerized condition upon being subjected to mechanical shear stresses. Low molecular weight PTFE, on the other hand, simply reaches its ultimate elongation at low stress and disintegrates into a lubricating (low coefficient of friction) powder while highly crystalline high molecular weight PTFE substantially transforms its morphological character under shear and forms an extensive network of fibers. This is most obvious in the case of aqueous, dispersion-polymerized, high molecular weight PTFE in which the growing polymer chains are highly organized into dense, tightly packed spheres or rods with a very high degree of crystallinity. The rod-shaped particles, when present, typically have a length to diameter ratio (L/D) of 2-3:1 and the diameter is typically on the order of 0.1 (micrometer or .mu.). The spherical particles typically have a diameter of approximately 0.2-0.3 .mu., as measured by light scattering. Because of their very high crystallinity and high molecular weight, it is possible for these particles to fibrillate into rod-like structures when subjected to a relatively low mechanical shear force, as illustrated in present examples and corresponding SEM photomicrographs, such as FIGS. 1 and 4, forming fibers having a very high L/D ratio. These PTFE fibers have the ability to form aggregated structures in which the rod-like aggregates of high molecular weight PTFE serve as a microfiber reinforcement within the polymer mass of fibrillated and unfibrillated PTFE. The presence of such structures results in an increase in the tensile modulus and strength of the polymer matrix in which they are present and for this reason may be referred to as a microfiber reinforcement. The ease with which such fiber formation occurs is such that one must take great care to control the level and direction of applied shear forces to avoid uncontrolled entanglement of propagating fibers which can result in physical unmanageability during subsequent processing. Melt viscosity is another commonly measured surrogate for molecular weight of polymers such as PTFE. Commonly measured at 380.degree. C., the melt viscosity of high molecular weight PTFE is typically about 10.sup.10 to 10.sup.12 poise. High molecular weight PTFE readily forms fibers of the type discussed above when at a high level of crystallinity. As the melt viscosity at 380.degree. C. decreases (indicating lower molecular weight), however, PTFE's ability to fibrillate falls off markedly. Below about 10.sup.9 poise, PTFE becomes a much more brittle, friable material.
Many attempts have been made in the prior art to combine PTFE with other polymeric compounds, such as elastomers, to form multicomponent systems. Fairly sophisticated processes have been developed to control the properties of such multicomponent systems, and skilled artisans have been able to enhance various desirable properties of final products created from such multicomponent systems. "Rubber-toughened" plastics are a good example of such an enhancement.
In creating these multicomponent systems, skilled artisans have used with some success "microparticulate polymers", i.e., emulsions and dispersions of elastomeric and plastic polymers, as coating and casting fluids. Various processes have been developed for applying such fluids, blended in the microparticulate state, enabling skilled artisans to thermally consolidate thin films in an extremely short duration of time and at surprisingly high temperatures, if necessary. Short interval thermal processing yields surprisingly compatible blends of microparticulates, even those with greatly disparate melt flows. For example, these processes have been used to combine polymers perceived to be non-extrudable due to their high viscosity or molecular weights or to combine non-melt-processible polymers, such as PTFE, with other more flowable polymeric components. Short interval thermal processing has also been used to combine materials with vastly different melting points (ranging from 150.degree. C. to 335.degree. C.). The absence of substantial mechanical shear during the high temperature phase of the thermal consolidation avoids mechanically-induced thermal deterioration of molecular weight in the materials, such as might occur during a melt-extrusion process.
Despite all of these efforts, the prior art has not been able to develop blended solid compositions containing fluoroplastics at particularly high useful levels into fluoroplastic/fluoroelastomer blends, while maintaining facile processibility of the blends.
Polymeric intermediates (for example, gum rubbers) must first be isolated before they can be compounded into a processible composition that can incorporate fillers, such as carbon or talc. The initial isolation of the polymeric intermediate generally involves the steps of coagulating the polymer from a polymerization medium, followed by washing the polymer, drying the polymer, and compacting the polymer into a slab. This polymeric slab is then mixed with desired curing additives and fillers on a high shear mill, such as a Banbury mill, keeping the mixture below temperatures that would initiate localized or premature cure of the rubber compound. Such premature cure is known in the art as "scorch." Other additives may be incorporated in a similar manner. Fillers are generally incorporated before the curatives because the incorporation process generates heat, which would cause scorch if the curative were present. The amount of such additives is limited because additional space in the compound is needed to accommodate the presence of the curative. Additives and curatives are added sequentially, which overall affects the workability of the gum. Once the additives and curatives are added, the compound is stored at a cool temperature until used. The compounded formulation is then typically "freshened" on a mill or calender and extruded at non-scorching conditions to yield a form that may be compression-molded, transfer-molded, or injection-molded to produce a shaped, cured part such as an "o"-ring or seal. Cured parts may then be demolded and post-cured at elevated temperatures to develop maximal mechanical properties or chemical resistance.
The prior art evidences substantial effort to identify desirable fillers and curing additives that improve end properties. PTFE has been identified as an excellent filler because of its desirable tribological properties. The prior art processes incorporating milled PTFE, however, have been limited to filler loadings below 25 parts per hundred rubber by weight to avoid problems both during milling and in subsequent processing. Furthermore, the prior art has also been constrained to employ low molecular weight PTFE micropowder, such as DuPont's MP 1000, as a filler. For this reason, users of such blends have not been able to exploit the benefits of high molecular weight PTFE related to its tendency to fibrillate under applied mechanical shear. Thus, the prior art processes have not generally been able to achieve a processible composition as a blended solid containing a high filler loading of PTFE, and particularly with high molecular weight PTFE. The potential benefits of a homogeneously distributed, high molecular weight PTFE at a high filler loading in the compositions have been, consequently, forgone.
The present invention achieves the desirable benefits discussed above through the use of an essentially non-fibrillating, low-shear isolation process that incorporates in excess of 25%, and typically 40-50%, and as high as 80% by weight of a plastic (particularly high molecular weight PTFE, a perfluoroplastic) into the rubber (based on total polymer weight). The process yields compositions containing the plastic PTFE in an unfibrillated form, yet the PTFE is fibrillatable and may be fibrillated by subsequent processing. The range of the substantial benefits obtained are described as follows.